I. Field of the Invention
The present invention relates to communication systems. More particularly, the present invention relates to the transmission of packetized data over circuit switched frame based data communication systems.
II. Description of the Related Art
Data Networks
Networks for interconnecting information processing entities are presently in widespread use. In general, networks provide for the communication of digital data between two or more data terminals or processing devices. Internetworks are essentially groups of individual networks coupled together so as to allow data interchange therebetween. The largest and most commonly known digital data network is the Internet.
Typical networks operate based on the transfer of discrete, quantized units of data called packets or frames. Thus, an entire file of data, such as an e-mail message, is segmented into a series of frames for transmission over the network. The actual data in each frame is attached to a series of headers associated with a set of protocol layers, described in additional detail below. Each protocol layer is devoted to handling one or more tasks involved with the transportation of data between terminals.
Most digital networks are comprised of many nodes or branch points. On its journey through the network, a frame may pass through a series of network nodes. The nodes may be generally categorized as repeaters, bridges, routers, switches or gateways based largely on 1) the highest protocol layer which is examined by the node, and 2) whether the node transforms the data between transport protocols. The nodes may be connected using a variety of different physical media referred to as links.
Network Protocols
An industry standard model describing seven protocol layers ranging from the physical layer to the application layer is defined by the Open Systems Interconnection (OSI) model. The layers are related hierarchically by level of abstraction with the physical layer being the lowest and the application layer being the highest. For instance, RS-232 is a physical layer protocol defining the electrical signal interface between terminals, whereas Simple Mail Transfer Protocol (SMTP) is a commonly used application layer protocol for sending e-mail over a network. A similar model, Transport Control Protocol/Internet Protocol (TCP/IP), is presently in widespread commercial and military use in data networking applications. The TCP/IP protocol suite (also known as a xe2x80x9cstackxe2x80x9d, due to the aforementioned layered construction) uses application, transport, internetwork, and network access layers. Overall, the TCP/IP suite provides a number of functions, including remote file transfer and copy, remote login capability, gateway and router support, electronic mail (STMP), and serial line communications via other existing protocols such as PPP or SLIP (described further below).
Generally speaking, Internetwork Protocol (IP) is implemented in all end systems and routers; it acts to relay or move a block of data on an internetwork from one host to another via one or more routers. Hence, it facilitates the delivery of data message packets (typically called xe2x80x9cdatagramsxe2x80x9d; see discussion of packet switching below) from two devices not directly connected. TCP is typically resident only in end systems; it essentially functions to track and deliver data to the appropriate application layer entity. Specifically, TCP avoids loss, damage, duplication, or misordering of datagrams that may result from the application of IP by way of checksums, sequence numbers in the TCP header, and other means. Additionally, security and access limitations may be applied via TCP. In the generic TCP/IP suite, the network layer uses existing network protocols (such as Ethernet, IEEE Standard 802, or X.25), and encompasses those protocols necessary to effectuate physical communication with other network nodes or entities. FIG. 1 illustrates the basic OSI and TCP/IP protocol suite models. IETF RFC 793 and 791 provide additional information on TCP and IP protocols, respectively.
Two levels of addressing are typically used in protocol suites such as TCP/IP. The first specifies the global internetwork (IP) address of a given host on a network, typically a 32-bit word. The second is unique within the host; i.e., it allows the host-to-host protocol (such as TCP) to deliver data to the proper application within a given host entity. This second address is commonly known as a port. Systems running TCP/IP typically have what is known as a kemal or internet routing table consisting of a series of entries, each entry containing multiple data fields. These fields include the destination IP address, a network mask, network xe2x80x9choppingxe2x80x9d address (i.e., the next machine which knows how to reach the ultimate destination of the data message), and the identity of the network interface device through which the datagram(s) must be sent to reach the next hop. Routing xe2x80x9cdaemonsxe2x80x9d initialize and dynamically update the kernal routing table by communicating with comparable entities in other systems to exchange routing information.
The Point-to-Point Protocol (PPP) is a link protocol which provides for the use of network applications within the TCP/IP suite over serial line interfaces (such as dial-up Internet connections) by linking the serial line(s) to the IP protocol driver. The PPP stack typically consists of multiple components, including an asynchronous high level data link control (HDLC) protocol, a link control protocol (LCP), an network control protocol (NCP), and authorization protocols. The HDLC layer is typically the lowest layer of the stack, and functions to provide framing for data packets, error detection, and frame identification for high level protocols. The LCP sits above the HDLC and dynamically determines transmission link characteristics (MRU and ACCM) and integrity. The NCP (for example, Internet Protocol Control Protocol {IPCP} for IP suites) carries out addressing functions relating to the PPP link. Note that multiple NCPs can run on the same PPP link. RFC 1661 and 1332 provide additional information on PPP and IPCP protocols, respectively.
The PPP operates generally as follows. In order to establish a point-to-point link, each end of the PPP link must first send LCP packets to configure and test the data link. After the link has been established and transmission features negotiated, PPP sends NCP packets to select and configure the operable NCPs. Once each of the chosen NCPs is configured, datagrams from each NCP can be sent over the link.
Wireless Link to a Data Network
With the advent of wireless communication techniques, data networks now provide the terminal user with greatly increased mobility. For example, a lap top computer may utilize a cellular telephone transceiver to connect to digital networks such as the internet without a wired connection to a physical data port. Numerous other data network applications over wireless links are possible, as described below.
FIG. 2 is a representation of a terrestrial wireless communication system 10. The system illustrated in FIG. 2 may use code division multiple access (CDMA), time division multiple access (TDMA), a combination of frequency hopping and TDMA (such as the Global System for Mobile Communications, or GSM) or other modulation and access techniques. FIG. 2 shows two remote units 10, 12, and two base station antenna 14. In reality wireless communication systems may have hundreds of thousands of remote units and many hundreds of base stations. In FIG. 2, the remote unit 10 is shown as a mobile telephone unit with a laptop computer 11 connected thereto. FIG. 2 also shows the personal digital assistant 12 in a standard cellular system. In the most general embodiment, the remote units may be any type of communication unit. For example, the remote units may be hand-held personal communication system (PCS) units, portable data units, or fixed location data units such as meter reading equipment. FIG. 2 shows a single wireless link 16 between the base stations 14 and the remote units 10 and 12, although it can be understood that such link may comprise a separate forward and reverse link. Base station transceiver/controller (BST/BSC) equipment 18 is provided to support each base station and interface with the mobile switching center (MSC) complex 20. The MSC complex 20 may contain one or more interworking functions (IWFs) as described in greater detail below. Ultimately, the cellular mobile units 10, 11 and 12 interface with land-based users 22 via a standard public switched telephone network (PSTN) switching device 24.
The following discussion assumes operation in accordance with the system described in TIA/EIA Interim Standard 95-B published by the Telephone Industry Association entitled xe2x80x9cMobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System,xe2x80x9d commonly referred to as IS-95. It can be appreciated, however, that the generic principles of the present invention can be directly applied to other wireless standards and types of data systemsxe2x80x94even those which may not be associated with a wireless link.
A remote unit which is used for voice communication transmits and receives frames of voice data over established traffic channels within a wireless link. As voice frames are transferred over a wireless link, it is not required that each frame passes without error over the link. Telephone quality voice is produced even with uncorrected frame error rates as high as 3 errors in 100 frames transferred. When confronted with an erroneously received frame, the physical layer simply discards the frame and replaces the frame with a frame erasure in order to maintain synchronization. Therefore, the physical link protocol as defined in IS-95 does not provide for the error-free transfer of frames.
In contrast to voice data, digital information data must typically be error-free in order to be useful. For CDMA data services, a new radio link protocol (RLP) layer is added to the protocol stack in order to substantially reduce the error rate exhibited by CDMA traffic channels. The RLP provides frames to and accepts frames (and frame erasures) from the physical link protocol. One primary function of the RLP is to provide error detection and correction.
A family of data transmission standards compatible with IS-95 has been adopted by the Cellular Telephone Industry Association. An early standard is described in TIA/EIA Interim Standard 99 (IS-99) and entitled xe2x80x9cData Services Option Standard for Wideband Spread Spectrum Digital Cellular System.xe2x80x9d Another more recent standard is described in IS-707 entitled xe2x80x9cData Service Options for Wideband Spread Spectrum Systems.xe2x80x9d IS-99 and IS-707 define, among other things, an RLP which provides the functionality just described.
IS-99 also defines the technical requirements for cellular Service Options xe2x80x9c4xe2x80x9d and xe2x80x9c5xe2x80x9d (SO4 and SO5) for asynchronous data service and digital facsimile service, respectively. In general, the term xe2x80x9cservice optionxe2x80x9d can be defined as a set of requirements which define the way in which traffic channel frames (forward and reverse link frames in the IS-95 context) are processed by the mobile units and cellular infrastructure. Specific service options are available to each individual mobile user based on the types of services requested by that user, the configuration of his mobile unit, and the capabilities of the local infrastructure. Numerous circuit switched service options, such as SO12, exist in addition to SO4 and SO5 referred to above; however, it will be appreciated that such other options are largely interchangeable with SO4 and SO5 for the purposes of this discussion.
FIG. 3 depicts the generic IS-99 Um (air interface) protocol stack, hereinafter referred to as the IS-99 stack. As shown in the Figure, this stack is conceptually comprised of several components including, inter alia, the circuit switched IWF stack 21, the circuit switched BSC/MSC stack 23, and the circuit switched mobile stack 25. The IS-99 stack describes the minimum TCP, IP, and PPP protocols for the base station and mobile unit. Note the various functional layers as previously described, including the physical layer (IS-95A), the RLP layer, the PPP (data link) layer, the TCP (transport) layer, and the IP (network) layer. Also depicted is the subnetwork dependent convergence function (SNDCF) sublayer, which performs compression/decompression on the headers of datagrams passed from/to the transport and network layers.
Circuit Switching Versus Packet Switching
Two approaches to call data switching and routing are commonly employed within modern communication systems: circuit switching and packet switching. A circuit switched network can be generally defined as one in which a dedicated communication channel is maintained for the entire duration of message or data transmission. Data from multiple sources can be transmitted over physical transmission links using well known techniques such as time-division multiplexing; however, the allocation of the link to a given user (or set of users) is fixed for the duration of the call, and only one destination address can be specified for each user. Circuit switched systems are considered inefficient from the standpoint that data is often transmitted over the dedicated circuit only a fraction of the time, thereby having large segments of unused channel capacity (even when multiplexed). Common circuit switched data applications in the wireless context include modem and facsimile transmissions. It should be noted that the concept of circuit switching is not discordant with the concept of data framing as previously discussed; that is, framed data may be (and routinely is) sent over a circuit switched network.
In contrast to circuit switching, packet switching techniques make use of the packetized nature of the transmitted data (datagrams) to allocate resources within the switching network, and create the potential for distributing the packetized data from a single source to multiple users. Typically, datagrams are sent from an originating or source device to the destination device via one or more dynamically allocated physical channels while being interleaved or multiplexed with datagrams from other sources and/or destinations (see prior discussion of network protocols). Such dynamic allocation is often statistical in nature rather than of a fixed relationship as in circuit switched networks. Typically each datagram contains addressing and sequence information, making it a largely autonomous unit of data. Accordingly, individual packets of the same message may be routed differently to the same destination. Ultimately, the individual packets are reassembled, resequenced (if required) and formatted by the destination switching center for delivery to the destination address. In the wireless context, TIA/EIA Interim Standards 657 xe2x80x9cPacket Data Service Options for Wideband Spread Spectrum Systemsxe2x80x9d and 707 xe2x80x9cData Service Options for Wideband Spread Spectrum Systems: Packet Data Servicesxe2x80x9d govern the application of packet data services to CDMA systems. Referring to the protocol stack paradigm previously discussed, an IS-657/707 data packet could consist of the TCP/IP transport and internetwork data, the PPP (or other link layer protocol) data, and the RLP data, although other constructs are possible. As the packet is processed by successively higher layers of a given protocol stack, the header information and data associated with that layer are stripped off to provide critical packet handling and formatting instructions for the payload data.
One significant result of using a packetized data architecture such as that described above is that only intermittent use of air interface system resources is required in transmitting/receiving the packet data message(s), thereby potentially allowing simultaneous voice and data transmission/reception from the same mobile unit. Furthermore, the packetized data may be transmitted between the various CDMA cell cites and the mobile switching center (MSC) using the same backhaul as voice data, thereby not requiring additional packet data handling infrastructure. Other advantages offered through the implementation of packetized data protocols are manifold. For example, wireless mobile telephones can be connected to the internet without the need for extra modem hardware. Existing worldwide web browser applications, such as the UP.LINK(trademark) product offered by Unwired Planet Inc., can connect to the internet (and ultimately the associated server) via the air interface to transmit a variety of different types of useful information to and from the mobile unit. Other devices such as personal computers (PCs) and personal digital assistants (PDAs) may also be connected to the internet via the mobile unit.
Despite the foregoing advantages, many cellular infrastructure manufacturers and service providers currently do not support CDMA packet data services in accordance with IS-657/707, and may not do so for a significant period of time. On the contrary, however, a plethora of existing and planned mobile end-user products and systems (such as the aforementioned Unwired Planet UP.LINK browser), utilize packetized data protocols. Existing digital wireless infrastructure, presently in widespread use throughout the world, lacks the intelligence necessary to recognize these packet data protocols without significant hardware and software modification. FIG. 4 shows a typical IS-95 base station/packet interworking function (IWF) architecture designed to support both IS-657/707 packet and circuit switched services. In the present context, the IWF is a communications interface device which allows interworking between outside switching and networking devices (such as PSTN entities) and the BSC/MSC. One embodiment of a circuit switched IWF is a pool of modems, although other types of data interfaces such as Ethernet or ISDN are frequently included within the IWF. The architecture depicted in FIG. 4 utilizes one or more IWF packet modules (IWF-P) 30 separate from those used for handling the circuit switched data (IWF-CS) 32. The IWF-P 30 routes packet data to a public switched data network (PSDN) 36 or equivalent, while the IWF-CS 32 routes circuit switched data (such as asynchronous data or digital facsimile calls) to a PSTN 24 or equivalent. Without an IWF-P module (and other related software and components), existing base station/MSC complexes 34 are incapable of differentiating a packet data call from a circuit switched call on existing cellular service options.
The most promising prior art method conceived to date to allow existing cellular infrastructure to recognize and process a packet data call without a separate IWF-P or service option involves the use of a separate IP/PPP protocol layer in conjunction with an IS-99 layer, as illustrated in FIG. 5. In this method, both the mobile unit 10, 12 and the IWP 32 utilize a separate IS-99 layer 40 through which all calls are processed over an existing circuit switched service option such as SO4. By inserting a special xe2x80x9cATxe2x80x9d command (such as ATDT#xxx, for example) in the transmitted IS-99 layer data of the packet call datagram, the system could be cued to route subsequent message payload data to the next higher protocol layer (PPP, in this instance) 42 as opposed to routing them to a modem as would occur for a normal circuit switched call. Data directed to this PPP layer 42 would then be routed to the internet and the ultimate destination address via the higher layer protocols such as TCP/IP 44 previously described. However, this method has several significant drawbacks, including 1) significant processor and memory capability is required to process the IS-99 and IP/PPP stacks within the mobile unit and IWF during a packet data call, which may already be operating at or near capacity; 2) additional protocol layers require additional synchronization time, thereby increasing call setup and connection time; 3) additional protocol layers require significant additional engineering of the software code in the stack(s); and 4) the presence of additional protocol layers in both the mobile unit and IWF necessitate the transmission of additional data bytes over the air interface, thereby reducing overall system capacity and increasing latency.
Hence, an improved architecture and data handling method is needed to allow conventional cellular infrastructure and mobile units to interface with packetized data servers and end-user products, especially in light of the ever-increasing commercial demand and widespread implementation of these products. Such architecture and method would ideally require only negligible modification to the existing infrastructure and mobile unit hardware and software, thereby permitting cellular service and equipment providers to upgrade to packet data handling capability in a rapid and cost-effective manner, and without impact on existing customer support, system data handling capacity, or performance. Furthermore, the implementation of such packet data capability through existing cellular service options would be highly desirable from, among others, a consumer cost perspective.
According to the present invention, an improved architecture and method for transmitting packetized digital data over a circuit switched digital communications system is provided.
In one embodiment of the invention, a method of data communication is described which allows packetized data to be transmitted over digital cellular communications infrastructure which normally only supports circuit switched applications through the proper construction of the call data and protocol stacks. Specifically, on packet data calls, the receiving entity (base station/IWF in the present example, although as further described herein, mobile devices may also employ the method of the present invention to receive and process packet data) initially treats the packetized data call as being of the circuit switched variety, thereby permitting further processing. However, based on identification information embedded in the received data packets, the IWF is able to discern that the call is in packet data format, and payload packet data is subsequently routed directly to and processed by only those portions of the protocol stack relevant to packet data transfer. In this fashion, the circuit switched only terminals may be used to rapidly process such xe2x80x9cpseudo-packetxe2x80x9d or Fast Circuit Switched (FCS) data calls as if they were conventional circuit switched calls without the need for a separate cellular service option for packet calls. This method provides the further benefit of allowing a cellular service provider to support a greater total number of voice and data calls, since less airtime and processing is required to transmit and process the packet data as opposed to other prior art methods. As will be further described herein, both mobile originated and mobile terminated packet data calls may be processed using the technique of the present invention.
In another embodiment of the invention, an improved cellular infrastructure and protocol stack architecture is disclosed which allows for handling and routing of packetized data over circuit switched systems in accordance with the above-described method. A single IWF module is used to handle both circuit switched and packet data calls, thereby reducing the hardware and software necessary to support packetized data capability. In one embodiment of the protocol stack, identification of the call as being packetized is performed at the IP layer without the need for processing by a separate IS-99 layer or additional PPP layer. Such reduction of the hardware and software necessary to process a pseudo-packet or FCS call has many benefits, including less synchronization and training time between the synchronous layers in the protocol stack (such as PPP layers and TCP), as well as reduced mobile unit and base station signal processing and memory capacity necessary to process a given call.
In still another embodiment of the invention, the dormancy feature of the mobile unit is supported on mobile-terminated packet data calls through the use of a second cellular service option. Specifically, upon identification of a given call as a packet data call, the BSC/MSC is instructed by the IWF to page the dormant mobile unit using a second existing service option (such as SO5 for digital facsimile data, or other comparable option) for packet data calls only, thereby bypassing any ambiguity associated with paging of the dormant mobile on SO4 used for normal asynchronous data calls (or similar).